Magnetic-field sensors are used in a variety of applications to sense ambient magnetic fields. Applications for such sensors include automotive control systems, geological- and space-positioning systems, and medical devices, to name just a few. Magnetic-field sensors can use a variety of different principles and mechanisms to sense magnetic fields. One type of magnetic-field sensor is an anisotropic magnetoresistive sensor (AMR sensor). AMR sensors rely on the anisotropic sensitivity of the resistivity of certain magnetic materials to implement electric or electronic circuits, which can then provide outputs representing properties of sensed ambient magnetic fields.
One type of AMR sensor includes a resistor-bridge circuit having resistors composed of such anisotropic magnetic materials. A discrete deposit of anisotropic magnetic material to form a resistor, also known as a magnetoresistor, typically has, as a magnetic property, a total anisotropy represented by a characteristic magnetic field, also known as the total anisotropy field, in a direction parallel to an easy axis of the total anisotropy, also known as the total anisotropy axis. The total anisotropy is a function of a first component, the technological anisotropy, depending on both the crystal structure and processing conditions of the material, and a second component, known as the shape anisotropy, depending on the shape of the deposit that forms the resistor. If the shape of the deposit that forms the resistor is an elongated strip, then the shape anisotropy axis is typically along the longitudinal axis of the strip. If no ambient fields are present, the total anisotropy causes the magnetization of the magnetoresistor to align itself parallel to the total anisotropy axis, in either of two mutually opposite directions along this axis.
A resistor so formed has an electrical resistance to the flow of current through the material dependent upon the angle between the flow of current and the direction of the magnetization existing in the material at a given time. If an ambient field is present, it rotates the angle of magnetization existing in the resistor material, with the greatest rotation, and the greatest change in resistivity of the magnetoresistor, being when the ambient field acts perpendicular to the total anisotropy axis, or along what is known as the sensitivity axis. The amount of magnetization rotation is in this case inversely proportional to the total anisotropy field if the ambient field is much smaller than the total anisotropy field. As the total anisotropy field is a constant, the signal output by the bridge is therefore representative of the sensed ambient field.
A problem with bridge-type AMR sensors occurs when it is desirable to implement a plurality of such sensors using anisotropic magnetoresistive materials of having only a single technological anisotropy axis. Such a scenario can occur, for example, when it is desirable to implement as a single integrated circuit a multi-axis AMR sensor to sense and output signals measuring several different orthogonal vector components of an ambient magnetic field. For various reasons, manufacturing an integrated circuit with anisotropic magnetoresistive materials of multiple technological anisotropy axes is both technologically difficult as well as costly. Therefore, integrated multi-axis AMR sensors are typically constrained, at least practically speaking, to including anisotropic magnetoresistive materials of only a single technological anisotropy axis.
Such a constraint results in significant problems, however, for the design and operation of multi-axis bridge-type AMR sensors. The sensitivity of anisotropic magnetoresistors to ambient magnetic fields depends in part on the technological anisotropy, as it is inversely proportional to the total anisotropy field and the sensitivity axis is perpendicular to total anisotropy axis. While one may override the technological anisotropy axis using a strong shape anisotropy field, resulting in a total anisotropy axis almost parallel to shape anisotropy axis, if the shape anisotropy axis is very different than the technological anisotropy axis, i.e., angled at, e.g., 90° to each other, the magnetization of the whole magnetoresistor is likely to no longer be uniform. Instead, it is possible that many different smaller areas, with various magnetization directions, will form within the magnetoresistor and its sensitivity thereby degraded. Even if this does not happen instantly, it may happen after even a small ambient field is experienced. Such a scenario increases in likelihood with an increasing angle between the technological and shape anisotropy axes. In other words, anisotropic magnetoresistive materials typically do not like to keep their magnetization parallel to the longitudinal axis of the magnetoresistive strip if the strip is perpendicular to technological anisotropy axis.
Therefore, generally speaking, bridge-type AMR sensors have their technological anisotropy axis aligned in some predetermined manner to the vector component of the ambient magnetic field that they measure. However, it is difficult for a single technological anisotropy axis to be aligned in a sensitivity-maximizing manner to more than one different orthogonal vector ambient field component, such as to both x-axis and y-axis ambient field components. Thus, multi-axis bridge-type integrated AMR sensors constrained to a single technological anisotropy axis may not be able to use the same sensor design for each sensor unit sensing a different ambient field vector component. Moreover, if a particular sensor unit of a multi-axis sensor does have its technological anisotropy aligned favorably with the vector component it is to measure, the other sensor units, for sensing other vector components of the magnetic field, may be unfavorably aligned to the technological anisotropy axis, and therefore may suffer performance degradation.
Therefore, there exists a need for multi-axis bridge-type integrated AMR sensors that can be manufactured using anisotropic magnetoresistive materials having only a single technological anisotropy axis, but which still provide good performance for separately sensing multiple orthogonal vector components of an ambient magnetic field.